Nitrogen oxides (NOx), which include nitric oxide (NO) and nitrogen dioxide (NO2), are formed during the high temperature and pressure combustion of an air and fuel mixture in an internal combustion engine. These oxides cause a number of concerns related to the environment, such as a source of ground-level ozone or smog, acid rain, excess aqueous nutrients, and can readily react with common organic chemicals, and even ozone, to form a wide variety of toxic products. Since the 1970's, government legislation has required increasing reductions of NOx in exhaust gas emissions.
To comply with increasingly stringent government mandates, industry has developed several NOx reduction technologies to treat post combustion exhaust, of which diesel oxidation catalyst (DOC) and selective catalytic reduction/reducer (SCR) technologies are actively pursued.
In addition to NOx reduction, governments have been imposing progressive mandates for reducing amounts of particulate matter (PM) in exhaust emissions. The diesel particulate matter filter (DPF) has been developed for exhaust aftertreatment systems to remove diesel particulate matter containing soot, unburned fuel, lubrication oil etc. from the exhaust gas.
A DPF typically includes a filter encased in a canister that is positioned in the diesel exhaust stream. The filter is designed to collect PM while allowing exhaust gases to pass through it. Types of DPFs include ceramic and silicon carbide materials, fiber wound cartridges, knitted fiber silica coils, wire mesh and sintered metals. DPFs have demonstrated reductions in PM by up to 90% or more and can be used together with a DOC to reduce HC, CO, and soluble organic fraction (SOF) PM in diesel exhaust.